1. Field of the Invention
The present invention relates to the use of coherent energy pulses from high powered lasers used in laser shock processing of solid materials, and more particularly, to an apparatus and method for clearing or providing a substantially debris free laser beam path during laser shock processing.
2. Description of the Related Art
Laser shock processing involves directing a pulse of coherent radiation to a piece of solid material to produce shock waves therein. The produced shock wave cold works the solid material to impart compressive residual stresses within the solid material. These compressive residual stresses improve the fatigue properties of the solid material.
Current laser shock processing utilizes two overlays: a transparent overlay (usually water), and an opaque layer (usually an oil based paint or black plastic tape). During processing, the laser beam is directed to pass through the transparent overlay and is absorbed by the opaque overlay, causing a rapid vaporization of the opaque overlay (plasma creation) and a generation of a high-amplitude shock wave. The shock wave cold works the surface of the part and creates deep compressive residual stresses, which provide an increase in fatigue properties of the workpiece. A workpiece is typically processed by processing a matrix of overlapping spots that cover the fatigue critical zone of the part.
During laser shock processing, some transparent and opaque overlay material becomes dislodged from the workpiece to form particulate matter or debris scattered through the gas surrounding the workpiece. Depending on the composition of the overlay material, the debris may be water droplets, water vapor, paint, or tape fragments. After a laser shot, the presence of debris expands to fill the environment which also contains the laser beam path and optics for the next laser shot. The debris or particulate matter within the laser beam path absorbs and scatters energy from the laser pulse.
One problem with current laser shock processing systems is that the presence of debris from previous laser shock processing cycles may interfere with the laser pulse of subsequent laser shock processing cycles. The debris or particulate matter residue located in the laser beam path absorbs and scatters energy from the laser pulse. As a result, there is an energy loss from the laser beam associated with the presence of debris within the laser beam path. Consequently, the amount of energy applied to the opaque layer is altered, and thereby, affects the amount of energy applied to the workpieces.
Another problem with the current system of laser shock processing is the increase in the density of debris or particulate matter within the laser beam path as the repetition rate of laser peening increases. In other words, there is a cumulative effect of debris residue present in the laser beam path when the pulse rate exceeds the time it takes for the debris to settle naturally out of the laser beam path.
Following a laser shock peening cycle, particulate matter or debris dislodged from the workpiece will settle over time. Typically, within four seconds, the debris will naturally settle from the laser beam path due to gravity. When the pulse repetition rate is xc2xc hertz or greater, there is insufficient time for the debris to settle from the laser beam path. At increasing repetition rates above xc2xd hertz, the cumulative effect of particulate matter or debris results in an ever-increasing loss of laser pulse energy due to the debris accumulation in the beam path. As a result, current laser shock processing systems are limited to repetition rates of approximately xc2xc hertz. Significant absorption and scatter of energy by the debris is not observed at these slower processing rates.
What is needed in the art is an apparatus and method for preventing debris or particulate matter from previous laser shock peening cycles from interfering with the laser beam path of subsequent laser shock peening cycles.
The present invention provides an apparatus and method for laser shock peening having a substantially debris-free laser beam path. In one embodiment, the invention includes a system for removing debris from the laser beam path. In an alternate embodiment, the present invention includes a system for preventing debris from entering the laser beam path.
The invention, in one form thereof, is an apparatus for improving the fatigue life of a workpiece. A laser is operatively associated with the workpiece. A laser beam path runs from the laser to the workpiece. A laser beam clearing means provides a substantially debris-free laser beam path.
The invention, in another form thereof, is an apparatus for improving the fatigue life of a workpiece through laser shock processing which limits laser shock processing debris from interfering with a pulse of coherent energy. A laser is operatively associated with the workpiece. A laser beam path runs from the laser to the workpiece. A shield prevents the laser shock processing debris from entering the laser beam path.
The invention, in yet another form thereof, is a method for providing a substantially debris-free path during laser shock processing. The method includes applying a transparent overlay to a workpiece. A substantially debris-free laser beam path is provided. A first laser pulse is directed to a workpiece. A second laser pulse is directed to a workpiece along the substantially debris-free laser beam path within four seconds of the first laser pulse.
One advantage of the present invention is the removal of debris or the prevention of debris entering into the laser beam path. The presence of debris within the laser beam path interferes with a pulse of coherent energy by absorbing and scattering energy from the laser pulse.
The invention, in another form thereof, is a method of cleaning a laser beam path of debris. The method includes the steps of directing a laser pulse to a workpiece and directing a flow of gas to remove debris from the laser beam path.
Another advantage of the present invention is the ability to increase the laser shock peening repetition rate. Prior to this invention, the repetition rate for a laser used in laser shock peening was limited due to the presence of debris within the laser beam path from previous cycles. Before one was able to fire the laser again, one had to wait for the debris caused by the previous firing cycle to naturally settle from the laser beam path. As a result, the repetition rate was limited to the time it takes for debris to naturally clear or settle from the laser beam path. The present invention either prevents debris from entering the laser beam path or removes debris that has entered the laser beam path. As a result, the present invention repetition rate is not limited due to debris from a previous laser shock peening cycle being present in the laser beam path.
Yet another advantage of the present invention is the ability to use a lower powered laser. Since the present invention provides a laser beam path substantially free from debris, there is substantially no debris to absorb and scatter energy from a pulse of laser energy. As a result, ever increasingly larger lasers are not needed in laser shock processing to overcome the effect of residue debris within a laser beam path.
Another advantage of the present invention is the achievement of more consistent results of compressive residual stresses imparted to a workpiece. The presence of debris within the laser beam path absorbs and scatters energy from the laser pulse. As a result, energy is lost from the laser pulse. Depending on the quantity of debris present in the laser beam path, the amount of energy lost will vary. The variability of energy loss, due to debris, affects the consistency of and reproducibility of compressive residual stress applied to a workpiece. The present invention allows for consistent compressive residual stresses applied to a workpiece by providing a predictable and consistent amount of energy applied to an opaque layer to form compressive residual stresses in the workpiece.